Method of production of high purity silica and ammonium fluoride

ABSTRACT

A method for producing high purity silica and ammonium fluoride from silicon tetrafluoride-containing gas wherein silicon tetrafluoride-containing gas from the acidulation of phosphorus-containing rock is recovered and the liquid entrainment is separated from the gas. The recovered gas is converted to an ammonium fluosilicate solution and is ammoniated to produce high purity silica and ammonium fluoride. The recovered gas can be converted to an ammonium fluosilicate solution either by absorbing the gas directly in a solution of ammonium fluoride or by first absorbing the gas in water to produce fluosilicic acid and then reacting the fluosilicic acid with ammonia or ammonium fluoride. In a second aspect, the invention provides a method of separately recovering high purity silica and ammonium fluoride from an impure aqueous fluosilicic acid solution by reacting the fluosilicic acid solution with ammonia or ammonium fluoride for a time and at a temperature sufficient to form ammonium fluosilicate solution, recovering solid ammonium fluosilicate from the solution, purifying the solid ammonium fluosilicate by dissolving the solid in high purity water or ammonium fluosilicate solution, recrystallizing solid ammonium fluosilicate, and recovering purified ammonium fluosilicate crystals from the solution, forming an aqueous solution of the purified ammonium fluosilicate crystals and ammoniating the solution for a time and at a temperature sufficient to precipitate silica, and separately recovering high purity silica and ammonium fluoride.

This application is a continuation of U.S. Pat. No. 4,981,664 entitled"Method of Production of High Purity Silica and Ammonium Fluoride."

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing high puritysilica and ammonium fluoride. More particularly, the invention relatesto a method of recovering high purity silica and ammonium fluoride froman impure fluoride-containing source.

2. Description of Related Art

Chemically-combined fluorine typically is present in substantially allphosphorus-containing rock (phosphate rock), such as fluorapatite andmineral phosphates. Generally, such phosphate rock contains as much as 4wt. percent fluorine. When phosphate rock is reacted with an acid, suchas sulfuric acid or hydrochloric acid, much of the fluorine value of therock is liberated as an undesired by-product in the gaseous phase, e.g.,as silicon tetrafluoride. Gaseous silicon tetrafluoride also isliberated when phosphoric acid is concentrated, as in the production ofphosphate-containing fertilizers or wet process phosphoric acid.

The noxious nature of silicon tetrafluoride requires that it be removedfrom the gaseous phase to avoid atmospheric pollution. Gaseous silicontetrafluoride usually is recovered by absorption in water; the gas ispassed through water absorption vessels or Venturi scrubbers. Absorptionof silicon tetrafluoride in water yields aqueous fluosilicic acidsolution and silica precipitate.

In the wet process method of making phosphoric acid, weak phosphoricacid typically is returned to the attack tank. However, in one methodknown in the art, the weak phosphoric acid is treated with sulfuricacid. The heat of dilution of the sulfuric acid is used to strip, asvapor, fluorine values from the dehydration of fluosilicic acid in theweak phosphoric acid. The fluorine is recovered primarily as silicontetrafluoride; some hydrogen fluoride is also recovered. This vapor isabsorbed in water, yielding fluosilicic acid and silica precipitate.

The market value of fluosilicic acid, and of fluosilicates derivedtherefrom, is not sufficiently high, however, to make their productionecononmically attractive. It has been an object of the prior art toutilize by-product silicon tetrafluoride to produce other productshaving greater market value. Therefore, attempts have been made todevelop commercially attractive uses for this by-product.

U.S. Pat. No. 3,271,107 discloses a process for producing silicapigments from fluosilicic acid, generated by absorbing silicontetrafluoride in water, by reacting fluosilicic acid with ammoniumhydroxide in two stages. In the first stage, a less-than-stoichiometricquantity of ammonium hydroxide is added with high agitation to product aslurry having a pH of between 6.0 and 8.0 and containing minute silicaparticles. The unreacted fluosilicic acid in this slurry then is reactedwith sufficient additional ammonium hydroxide to provide a final pHbetween about 8.3 and 9.0. Pigment quality silica precipitate then isseparated from the slurry.

U.S. Pat. No. 3,021,194 discloses a process for producing ammoniumbifluoride from fluosilicic acid and ammonium fluoride purportedlywithout undue loss of ammonia or fluorine. Concentrated fluosilicic acidis reacted with ammonium fluoride, or a mixture of ammonium fluoride andsodium or potassium fluoride, to produce aqueous ammonium acid fluoride(ammonium bifluoride) solution and solid alkali fluosilicate, includingammonium fluosilicate. After separating the solution from the solidalkali fluosilicates, solid ammonium bifluoride is recovered byevaporatively concentrating the solution. Alkali metal fluosilicates canbe recovered and sold, or can be converted to alkali fluorides byreaction with additional ammonia. Ammonium fluoride is produced andhydrated silica is precipitated by this ammoniation. The silica isindicated for use as a filler, a flatting agent, or as an insecticideprovided it contains some sodium fluoride.

Certain uses of silica require very high purity material. For example,silica used in the encapsulation or packaging of electronic computerchips must have extremely low levels of metal impurities. Typical ofthese uses is in very large scale integrate (VLSI) microchipapplications, where chip manufacturers require silica having extremelylow concentrations of certain radioactive elements. For example, uraniumand thorium concentrations must be on the order of less than 1 part perbillion (ppb). The maximum acceptable level of ionic impurities,including cations such as aluminum, boron, calcium, cobalt, chromium,copper, iron, potassium, magnesium, manganese, sodium, nickel, vanadium,and zinc, and anions containing phosphorus and sulfur, also is less than10 parts per million (ppm), and often is below 1 part per million.

Other uses for high purity silica material include precision laseroptics, fiber optics, and advanced ceramics, including diffusion tubesand crucibles. Presently, these requirements are satisfied predominantlyby natural silica sources such as quartz. Unfortunately, prior artprocesses for recovering silica from contaminated fluosilicic acidstarting materials have not been satisfactory for producing a productsatisfying these stringent purity requirements.

U.S. Pat. NO. 4,465,657, for example, discloses a process for producinga purified silica from impure fluosilicic acid which basically uses theprocedure of the earlier U.S. Pat. No. 3,271,107. Fluosilicic acid isreacted in a first step with a less-than-stoichiometric quantity ofammonium hydroxide to convert some of the acid to ammonium fluoride andsilica. The silica precipitate thus produced removes metal ionimpurities, presumably at least in part by adsorption, from the residualfluosilicic acid solution. The silica precipitate then is separated, andthe remaining solution having a lower level of impurities is reacted ina second stage with additional ammonium hydroxide to produce a purifiedsilica precipitate. Optionally, the residual fluosilicic acid solutionfrom the first precipitation stage may be treated with an ion exchangeor chelating agent to purify the solution further prior to the formationof the silica precipitate in the second precipitation stage.

A particular drawback of this procedure is that from 40 to 75 percent ofthe available silica in the fluosilicic acid is used as the vehicle forremoving impurities. Thus, only 25 to 60 percent of the silica values ofthe fluosilicic acid actually can be recovered in a purified form.Moreover, there is a tacit admission that the two step process does notproduce a satisfactory product since it is preferred to treat thesolution from the first step process with an ion exchange or chelatingagent prior to the second precipitation step.

European Patent Application 0,113,137 attempts to avoid the loss inyield of U.S. Pat. No. 4,456,657 by adding a chelating agent directly tothe impure fluosilicate acid solution to improve the purity of thesilica by sequestering or chelating multivalent metal ions in thesolution before ammoniation. Ion exchange also has been used for thesame purpose. However, these techniques tend to introduce otherimpurities, such as alkali metal ions, into the precipitated silica.Additionally, these prior art purification processes rely upon cationicexchangers and metal chelating agents and thus cannot satisfactorilyremove the phosphorus and sulfur impurities generally present as anionicspecies (SO₄ ⁻² and PO₄ ⁻³) in the fluosilicic acid by-product solutionstypically recovered from the acidulation of phosphate rock. Nor cananionic exchange agents be used because the anionic exchange agentssignificantly decrease the recovery of silica.

High-purity ammonium fluoride is useful as a precursor for making anoxide etchant for electronic applications. Simple evaporation of anaqueous solution of ammonium fluoride liberates ammonia and formsammonium bifluoride, NH₄ FHF, or NH₄ HF₂. Alternatively, ammoniumfluoride is also useful as an ammonium source for diammonium phosphate.

Accordingly, it is an object of the present invention to provide amethod for recovering high purity silica and ammonium fluoride from theby-products obtained by the acidulation of phosphate rock.

It is another object of the invention to provide a method for producinghigh quality silica having metal impurity content below 10 ppm, andpreferably below 1 ppm, and having radioactive element concentrationsbelow 1 ppb.

It is also an object of this invention to provide a method for producinghigh-purity ammonium fluoride solution.

SUMMARY OF THE INVENTION

In accordance with these and other objects, this invention provides in afirst aspect a method for producing high purity silica and ammoniumfluoride from silicon tetrafluoride-containing gas, particularly the gasgenerated by acidulation, for example with concentrated sulfuric acid,of weak phosphoric acid from the gypsum filtration step of the wetphosphoric acid process, said method comprising:

a. recovering silicon tetrafluoride-containing gas from the acidulationof a fluorine-containing phosphorus source;

b. separating liquid entrainment from the gas;

c. converting the gas recovered from step (b) to an ammoniumfluosilicate solution; and

d. ammoniating said ammonium fluosilicate solution to produce highpurity silica and ammonium fluoride.

The gas recovered from step (b) can be converted to an ammoniumfluosilicate solution by absorbing the gas directly in a solution ofammonium fluoride or by first absorbing the gas in water to producefluosilicic acid and then reacting the fluosilicic acid with ammonia orammonium fluoride.

In a second aspect, the present invention provides a method ofseparately recovering high purity silica and ammonium fluoride from animpure aqueous fluosilicic acid solution comprising:

a. reacting the fluosilicic acid solution with ammonia or ammoniumfluoride for a time and at a temperature sufficient to form ammoniumfluosilicate;

b. recovering solid ammonium fluosilicate from the ammoniated solution;

c. purifying the solid ammonium fluosilicate by dissolving the solid inhigh purity water or a high purity unsaturated ammonium fluosilicatesolution, recrystallizing solid ammonium fluosilicate, and recoveringpurified ammonium fluosilicate crystals from the solution;

d. forming an aqueous solution of the purified ammonium fluosilicatecrystals and ammoniating the solution for a time and at a temperaturesufficient to precipitate silica; and

e. separately recovering high purity silica and ammonium fluoride.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the production of ammonium fluosilicatesolution in accordance with the first aspect of the invention.

FIG. 2 illustrates an alternative embodiment for production of ammoniumfluosilicate solution.

FIG. 3 is a schematic diagram for the method of the second aspect of theinvention.

In the drawings, like identifiers are sued to identify like parts.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of producing high puritysilica and ammonium fluoride from impure silicontetrafluoride-containing gas, particularly the gas generated byacidulation, for example with concentrated sulfuric acid, of weakphosphoric acid from the gypsum filtration step of the wet phosphoricacid process. It has been discovered that high purity silica andammonium fluoride can be produced from this source of impure silicontetrafluoride-containing gas using a process which has as one keyfeature the initial separation of liquid entrainment from the gas. Ithas been found, quite surprisingly, that the majority of the impuritiespresent in the silicon tetrafluoride-containing gas recovered from theattack tank is removed with the liquid droplets, simply by eliminatingthe entrained liquid from the gas.

In one embodiment, the silicon tetrafluoride in the gas then ishydrolyzed by absorption in water to produce silica and fluosilicic acidsolution. Thereafter, the precipitated silica, which typically adsorbsadditional impurities, is separated from the fluosilicic acid solution.The solution then is reacted with ammonium fluoride or ammonia toproduce pure ammonium fluosilicate. In an alternative embodiment, thecleaned gaseous silicon tetrafluoride is absorbed in an aqueous ammoniumfluoride solution to produce ammonium fluosilicate solution directly.

Ammonium fluosilicate crystals recovered from ammonium fluosilicatesolution produced by either embodiment preferably is recrystallized atleast once to enrich its purity. When reacted with additional ammonia,this high purity ammonium fluosilicate yields high purity silica andhigh purity ammonium fluoride products. The method of the invention thusproduces valuable high purity silica and ammonium fluoride from anabundant, inexpensive, waste by-product obtained, for example, from theacidulation of phosphorus-containing rock.

The term "high purity silica" refers to silica containing metal impurityconcentrations of less than about 10 ppm per metal and individualnon-metallic impurity concentrations, especially sulfur and phosphorusconcentrations, of less than about 10 ppm. Preferably, the silica hasmetallic impurity concentrations of less than about 1 ppm andnon-metallic impurity concentrations of less than about 5 ppm. The term"high purity silica" also encompasses silica having uranium and thoriumconcentrations of less than about 1 ppb.

The term "high purity ammonium fluoride" refers to ammonium fluoridesolution which, at a concentration of 40 wt. percent, contains metalimpurity concentrations of less than about 1 ppm and non-metallicimpurity concentrations of less than about 5 ppm. Preferably, theammonium fluoride has metallic impurity concentrations of less thanabout 0.1 ppm and non-metallic impurity concentrations of less thanabout 0.5 ppm.

"High purity, " when used to describe a water source or other solutionused in carrying out the present invention, means the material has asufficiently low impurity level that any silica or ammonium fluoridesolution ultimately recovered in accordance with the process of thepresent invention constitutes high purity product as hereinabovedefined.

Silicon tetrafluoride-containing gas obtained from the acidulation ofphosphorus-containing rock and weak phosphoric acid from the gypsumfiltration step of the wet phosphoric acid process, particularlyacidulation with concentrated sulfuric acid, often is combined withadditional gas obtained from the evaporative concentration of wetprocess acid and typically is contaminated with multivalent cations andother impurities, such as sulfate and phosphate anions. It has beendetermined that the majority of these impurities actually are present ina liquid phase which is entrained as small liquid droplets in the gas.Thus, a majority of the impurities, and particularly the cationicimpurities, advantageously can be removed from the silicontetrafluoride-containing gas stream A to a surprisingly large extentsimply and conveniently by using an initial gas-liquid separation 1, asillustrated in FIGS. 1 and 2.

Liquid phase L is separated from the gas by using any of the techniquesand equipment available in the prior art for scrubbing or cleaningliquid droplets form a gas, such as cyclonic, impingement, orelectrostatic separation. Demisting, i.e., causing the gas to passthrough a porous filter-like element along a tortuous flow path so thatentrained liquid droplets are separated by impingement from the gasflow, is particularly useful. By this separation, metal cations absorbedin the liquid are removed from the gaseous silicontetrafluoride-containing stream. Unfortunately, some impurities,including sulfate and phosphate anionic impurities, may not be absorbedin the liquid which is separated from the silicontetrafluoride-containing gas, and thus remain in the gas phase.

Cleaned silicon tetrafluoride-containing gas stream B recovered from theinitial separation then is converted to an ammonium fluosilicatesolution by one of two alternative procedures. In one case, illustratedin FIG. 1, the gas is absorbed in high purity water to producefluosilicic acid which thereafter is reacted with ammonia or ammoniumfluoride to produce the desired ammonium fluosilicate solution. In analternative embodiment illustrated in FIG. 2, the gas is absorbeddirectly in a high purity aqueous ammonium fluoride solution to yieldthe desired ammonium fluosilicate solution. These absorbents are verypure, and should not contain any impurities which would contaminate theultimate silica and ammonium fluoride products. Water used to absorb thecleaned silicon tetrafluoride-containing gas or to make the aqueoussolution of ammonium fluoride therefore should be essentially free ofthe above-mentioned impurities.

Turning now to FIG. 1 and in accordance with the first alternative,cleaned silicon tetrafluoride-containing gas B is absorbed in water W inabsorber 2 to produce fluosilicic acid and silica (together, stream C)in accordance with the following reaction:

    3 SiF.sub.4 +2 H.sub.2 O → 2 H.sub.2 SiF.sub.6 +SiO.sub.2(I)

The absorption reaction occurs at standard conditions known to thoseskilled in the art. The concentration of fluosilicic acid in stream Cwhich can be produced in this manner typically ranges up to about 35 wt.percent, preferably between about 10 and 30 wt. percent. The reactantsand products are very acidic, so equipment line with acid-resistantinert materials should be used, thus helping to avoid undesirable metalcation contamination in the product.

Silica (SiO₂) P₁ which precipitates in the reaction is separated inseparator 3 from the fluosilicic acid solution by filtering, bycentrifugation, or by any suitable separation technique known to thoseskilled in the art. Thereafter, thus-separated fluosilicic acid D isreacted in reactor 4 in accordance with one of the following tworeactions to produce ammonium fluosilicate solution E:

    H.sub.2 SiF.sub.6 +2 NH.sub.4 F → (NH.sub.4).sub.2 SiF.sub.6 +2 HF(IIa)

    H.sub.2 SiF.sub.6 +2 NH.sub.3 → (NH.sub.4).sub.2 SiF.sub.6(IIb)

Reaction II (i.e., either IIa or IIb) typically is carried out at atemperature between about 25+ and 85° C., preferably between about 50°and 60° C.

Reaction IIa is carried out by adding at least about a stoichiometricamount of ammonium fluoride R to the fluosilicic acid D in reactor 4.Failure to add a stoichiometric quantity of ammonium fluoride results indiminution of yield, as all the silicon and fluorine in the fluosilicicacid will not be recovered. An excess of ammonium fluoride improvesrecovery of fluorine because the double salt NH₄ F (NH₄)₂ SiF₆ isformed, but requires recycle of solution. Also, ammoniation of thedouble salt may be more difficult than ammoniation of ammoniumfluosilicate solution. Thus, although any quantity of ammonium fluoridecan be used, the preferred quantity of ammonium fluoride is betweenabout 100 percent and 120 percent, and more preferably is between about100 and 100 percent, of the stoichiometric quantity.

In using reaction IIb, ammonia R is slowly sparged into the fluosilicicacid solution in reactor 4 until the first sign of silica formation isobserved. A less preferred method is to add the fluosilicic acidsolution to an ammonium solution which typically has a concentration ofabout 30 percent. Silica forms when ammonium fluosilicate is ammoniated,so incipient formation of silica indicates that all the fluosilicic acidhas been converted to ammonia fluosilicate. The silica P₂ thus formed isremoved from the warm ammonium fluosilicate solution F in separator 5prior to cooling the solution to recover solid ammonium fluosilicate.Any suitable separation technique may be used.

Preferably, cleaned silicon tetrafluoride-containing gas B is absorbedin aqueous ammonium fluoride solution Q in absorber 2 according to thesecond alternative illustrated schematically in FIG. 2, in accordancewith the following reaction:

    SiF.sub.4 +2 NH.sub.4 F(aq) → (NH.sub.4).sub.2 SiF.sub.6(III)

As required for reaction I above, equipment should be lined withacid-resistant inert materials to avoid undesirable metal contamination.

Reaction III is exothermic, so cooling may be required, depending uponthe equipment design. Typically, the temperature at which this reactionis carried out is allowed to rise from the ambient temperature of theabsorbent solution to a temperature below its boiling point. Preferably,the temperature is maintained between the freezing point of the solutionand about 85° C. With this approach, an ammonium fluosilicate solutionis produced directly from the cleaned gas.

Ammonium fluosilicate produced by the combination of reactions I and IIgenerally is purer than ammonium fluosilicate produced directly byreaction III, because some impurities tend to be adsorbed by the silicaprecipitated in reaction I. However, the stoichiometry of thesealternative reaction paths indicates that the yield of high puritysilica and ammonium fluoride should be higher for reaction III than forthe combination of reactions I and II. (Silica produced in reaction I isnot generally suitable as a high purity product.) Those skilled in theart can resolve the economic balance of yield and product purity againstthe cost of utilizing subsequent purification steps.

Turning now to FIG. 3, when absorption according to reactions II or IIIis complete, the temperature of ammonium fluosilicate solution F islowered to about 15° C. in chiller 6 to produce solid ammoniumfluosilicate by crystalliztion. Preferably, the solution is cooled tobelow about 10° C., and more preferably to below about 5° C. in order tomaximize the quantity of ammonium fluosilicate recovered and minimizethe residual concentration of ammonium fluosilicate in the motherliquor. Procedures suitable for cooling the ammonium fluosilicatesolution will be apparent to those skilled in the art. The solubility ofammonium fluosilicate in water ranges from about 11 percent at 0° C. toabout 38 percent at 100° C. Thus, lowering the temperature of thesolution lowers the saturation concentration and causes excess ammoniumfluosilicate at such lowered temperature to crystallize.

Through proper crystallization of the ammonium fluosilicate, many of theimpurities can be eliminated from the desired product. Crystallizationis effective for separating the crystallized solid from both cationicmetal impurities and anionic mineral impurities, i.e. sulfate andphosphate anions, present in the solution. However, care should beexercised in controlling the rate of cooling and thus the rate ofcrystallization.

Rapid crystallization of the ammonium fluosilicate results in aless-pure crystalline product because impurities tend to be trapped withthe crystal lattice. Rapid crystallization is more prone to occur if thelocal concentration of reactants is too high. Less inclusion occurs atreasonable crystallization rates. Methods for controlling thecrystallization rate will be apparent to those skilled in the art.

After the ammonium fluosilicate solution has been cooled and solidammonium fluosilicate has crystallized from the solution, the solids Hare separated from the solution G in separator 7. Simple filtration andother separation methods known in the art are suitable. Thus-separatedammonium fluosilicate crystals H then may be washed with a solvent inwhich ammonium fluosilicate is non-soluble, such as acetone, or is onlyslightly soluble, such as alcohol. Very cold water may be used if thetime of exposure of the crystals to the water is minimized. Although useof water avoids the use of organic solvent, it requires that care beexercised to prevent product loss because the crystals are soluble inwater. This washing removes surface contamination while the lowsolubility of ammonium fluosilicate in the solvent minimizes productloss. Of course, the wash fluid should be of high purity.

Ammonium fluosilicate crystals may be purified further byrecrystallization in recrystallizer 8. To effect recrystallization, thecrystals H first are redissolved in pure liquid water or unsaturatedammonium fluosilicate solution S₁ at an elevated temperature. Thesolubility of ammonium fluosilicate in water increases as temperatureincreases. Therefore, the fluid S₁ in which the ammonium fluosilicate isinitially dissolved should be as hot as possible. The temperature of theammonium fluosilicate solution then is lowered gradually to less thanabout 15° C., preferably to less than about 10° C., more preferably toless than about 5° C., to recrystallize a more pure solid ammoniumfluosilicate I. As above, careful control of the crystallization rate isimportant to maximize the purity enrichment. Depending on the desiredpurity, recrystallization may be carried out any number of times, withthe reformed crystals being removed from the solution each time. Thoseskilled in the art can balance economics of initial fluid temperature,cooling conditions, and number of recrystallization steps required toyield a desired purity.

At each separation, the solution S₂ remaining after therecrystallization may be recycle or reused. To discard the entiresolution after one recrystallization would be quire wasteful, becausethe solution will be saturated in ammonium fluosilicate at the finaltemperature. Typically, solution S₂ will have an ammonium fluosilicateconcentration of at least about 11 wt percent. Thus, the solution shouldbe recycled for further use. Of course, a fraction of the residualfluosilicate solution must be purged to maintain the concentrations ofimpurities in the recycle at acceptable levels. Thus a small make upstream may be required. Such techniques are known in the art.

Once the desired ammonium fluosilicate purity has been attained, thepurified crystals are redissolved in water to yield an aqueous solutionof ammonium fluosilicate T. This ammonium fluosilicate solution then isammoniated in ammoniator 9 by reaction with ammonium J to precipitatehigh purity silica and yield a high purity ammonium fluoride liquor inaccordance with the following reaction:

    (NH.sub.4).sub.2 SiF.sub.6 +4 NH.sub.3 +2 H.sub.2 O → SiO.sub.2 +6 NH.sub.4 F                                                (IV)

The temperature of the solution during the reaction typically may varyfrom about 25° to 85° C. The concentration of ammonium fluosilicate inthe aqueous solution can range up to about 32 percent, generally isbetween abut 3 and 25 percent, and most often is between about 10 and 25percent. Of course, the concentration of ammonium fluosilicate cannotexceed the saturation concentration at the selected temperature. Thus,if a solution temperature of 25° C. is utilized, the ammoniumfluosilicate concentration cannot exceed about 18 wt percent. Highconcentrations of ammonium fluosilicate are preferred if the desiredproduct is the high purity ammonium fluoride liquor and a moreconcentrated ammonium fluoride solution. However, lower concentrationsare preferred if silica quality is more important.

It has been discovered that the quantity of ammonium fluosilicatecrystals which can be treated can be maximized by first introducingammonia while maintaining the solution temperature between about 75°-85°C. until the ammonium fluosilicate concentration is reduced to betweenabout 5 and 10 wt. percent. Thereafter, the temperature preferably islowered to between about 40°-60 ° C., preferably between about 45°-55°C., for the remainder of the reaction.

This two-stage ammoniation is advantageous because it affords theopportunity to prepare an ammonium fluoride solution having aconcentration of about 40 wt percent. To prepare a 40 percent ammoniumfluoride solution requires use of a 32 wt percent ammonium fluosilicatesolution. However, this requires that the temperature be maintained at75°-85° C., i.e., at a temperature at which an ammonium fluosilicateconcentration of at least about 32 wt percent can be maintained. At thistemperature, introduction of a concentration of ammonia sufficient toconvert the ammonium fluosilicate to silica and ammonium fluoride is notpossible because of excessive loss of ammonia at higher temperature.Thus, after the ammonium fluosilicate concentration has been lowered tobetween about 5 and 10 wt percent, the solution temperature is loweredso that the stoichiometric quantity of ammonia can be introduced.

Ammonia can be bubbled through the aqueous ammonium fluosilicatesolution until all ammonium fluosilicate is converted to silica andammonium fluoride. As the reactants are depleted, the rate of silicaprecipitation will decrease, and will cease completely when the ammoniumfluosilicate is completely consumed. Typically, ammonia can be added tothe ammonium fluosilicate solution at a rate such that ammoniumfluosilicate exhaustion will occur in less than about 30 minutes.

The amount of ammonia utilized is at least about the stoichiometricquantity required to complete reaction IV. Preferably, an excess ofbetween about 5-30 percent, more preferably between about 10-30 percent,and most preferably, between about 20-30 percent of the stoichiometricamount of ammonia is used to ensure that the ammonium fluosilicate isexhausted. Therefore, the quantity of ammonia utilized is at least about100 percent, preferably is between about 105-130 percent, morepreferably is between about 110-130 percent, and most preferably isbetween about 120 and 130 percent of the stoichiometrically requiredquantity.

The conditions under which the ammoniation described in reaction IV iscarried out affect the properties of this silica precipitate. Thestoichiometry, concentration, and temperature all affect the propertiesof the silica.

Silica precipitate N can be separated from the ammonium fluoridesolution M in separator 10 by any suitable separation technique, such asfiltration, which is known in the art. A suitable separation should notintroduce impurities into the solution or onto the silica. Recoveredsilica N may be washed with purified water or other suitable solvents toremove residual ammonium fluoride solution from the silica.

Silica precipitate N and ammonium fluoride solution M produced by thisreaction will both have a high purity. The high purity ammonium fluorideproduct M of reaction IV may be recycled as a reactant in reactions IIaand III. Recycle of this high purity ammonium fluoride does notintroduce additional impurities at these stages, and therefore mayreduce the number of recrystallization steps necessary to achieve apreselected purity. The high purity ammonium fluoride solution recoveredin this process also is useful as an etchant e.g. in the manufacture ofintegrated circuit devices.

The following examples are intended to further illustrate the invention,not to limit the invention in any way. The invention is limited only bythe scope of the appended claims.

EXAMPLES 1 AND 2

Five thousand grams of an impure 25 wt. percent fluosilicic acidsolution was obtained from a wet process phosphoric acid plant. Thefluosilicic acid was prepared by absorbing silicon tetrafluoride gasexiting the attack tank directly in water without removing any liquidentrainment. The acid, which had the impurities listed in the secondcolumn of Table 1, was mixed with a stoichiometric quantity of ammoniumfluoride (Aldrich Company, Reagent grade) in a high purity silicacrucible. Ammonium fluosilicate precipitated during the mixing and wasseparated by decanting.

The separated ammonium fluosilicate then was added to pure water anddissolved by raising the temperature of the solution to its boilingpoint. This solution then was cooled to about 2° C. to recrystallizeammonium fluosilicate. This recrystallization procedure was performedthree additional times. Throughout recrystallization, the quantity ofwater was carefully controlled to maintain the concentration of ammoniumfluosilicate at just under 38 wt. percent to minimize the loss bydissolution in the mother liquor.

Ammonium fluosilicate crystals recovered from the last recrystallizationstep were redissolved in purified water without washing. Ammonia, in 20%excess, was charged into the ammonium fluosilicate solution toprecipitate silica. The silica was separated from the mother liquor byfiltration, and washed four times with 1.5 liters of purified water. Thewet silica cake was placed in a high purity silica crucible and dried ina Teflon®-lined vacuum oven at 120° C. and 15 torr overnight.

Two hundred grams of silica product was thus obtained. This isequivalent to a 30% yield. The concentration of impurities in thepurified silica was determined by inductively coupled plasma (ICP)atomic emission procedures. ICP is a technique used in analyzing traceimpurities. The impurities content in two distinct samples of thethus-purified silica also are presented in Table 1.

This example illustrates the effectiveness of recrystallization of thepurity of silica produced by ammoniation of fluosilicic acid.

COMPARATIVE EXAMPLE 1

For comparison purposes, the impurity content of a silica produced byhydrolysis of electronic grade, very high purity silicon tetrafluoridein water was measured. Gaseous silicon tetrafluoride from a gas cylinderwas bubbled through high purity deionized water to form silica inaccordance with reaction 1 described above. Again the impurities contentof the silica was measured by ICP.

The impurity data is presented in Table 1. The high purity silicasamples produced in Examples 1 and 2 from an impure fluosilicic acid andoriginally from an impure source of silicon tetrafluoride had puritieswhich are substantially equivalent to the purity silica produced byhydrolysis of a very high purity electronic grade silicon tetrafluoride.

                  TABLE 1                                                         ______________________________________                                               Impure                       Comparative                               Element                                                                              25% H.sub.2 SiF.sub.6                                                                    Example 1 Example 2                                                                             Example 1                                 Detected                                                                             (ppm)      (ppm)     (ppm)   (ppm)                                     ______________________________________                                        Al     4.0        0.2       0.1     0.3                                       B      0.1        0.1       0.1     0.1                                       Ca     --         1.3       0.6     0.1                                       Co     --         T         T       T                                         Cr     6.0        T         0.1     T                                         Cu     --         T         T       T                                         Fe     34.0       0.9       0.6     0.3                                       K      --         0.1       0.2     0.1                                       Mg     --         0.4       0.8     0.6                                       Mn     --         T         T       T                                         Na     --         T         T       0.2                                       Ni     4.3        0.1       0.1     T                                         P      0.9        0.2       0.1     0.1                                       V      0.1        T         --      T                                         Zn     --         0.2       --      0.1                                       S      55.0       0.3       --      1.3                                       Radioactive elements, ppb                                                     U      8           0.45     1       --                                        Th     --          0.46     --      --                                        ______________________________________                                         T = less than or equal to 0.05 ppm.                                      

EXAMPLE 3

Weak phosphoric acid recycled from the gypsum filtration step of a wetphosphoric acid process was mixed with concentrated sulfuric acid (92-98percent). The dehydration of fluosilicic acid contained in the weakphosphoric acid caused fluosilicic acid to decompose into gaseoussilicon tetrafluoride. This process was further facilitated by the heatof dilution of sulfuric acid. Air was purged through the mixed acid as acarrier gas. The air-silicon tetrafluoride mixture was forced throughfilters to remove entrained liquid droplets, then was bubbled into a 14percent ammonium fluoride solution to form a solution of ammoniumfluosilicate having an ammonium fluosilicate concentration of 25percent.

One thousand two hundred fifty kg of the 25 percent ammoniumfluosilicate solution was cooled to 10° C. to crystallize ammoniumfluosilicate. The ammonium fluosilicate crystals were separated from themother liquor with a centrifugal filter and were rinsed briefly withcold deionized water. Analyses of the 150 kg of ammonium fluosilicatecrystals and the 1,100 kg of mother liquor are summarized in Table 2.

The ammonium fluosilicate crystals were dissolved in 360 kg of deionizedwater, and this solution was kept at a temperature above 85° C. toprevent precipitation of the salt. Ammonia was charged into the solutionto decompose the ammonium fluosilicate into silica and ammoniumfluoride. The reaction temperature was maintained at over 80° C. untilthe concentration of ammonium fluosilicate was less than about 5percent. Then, the reaction medium was cooled to about 45°-50° C. tofacilitate ammonia absorption into the solution. When the final NH₃ /Fmole ratio reached about 1.2-1.3, the ammonia was stopped and thereaction was allowed to continue without addition of ammonia for about30 minutes. The slurry then was pumped into a centrifugal filter toseparate silica and ammonium fluoride solution. After washing anddrying, 45 kg of high purity silica and 460 kg high purity ammoniumfluoride were recovered.

The mother liquor from the crystallization step was ammoniated in asimilar manner. However, because the ammonium fluosilicate concentration(14.5 percent) did not require that the temperature exceed about 50° C.,single-stage ammoniation was utilized. Silica of lower, but acceptable,quality was recovered (50kg). The ammonium fluoride solution wasdiscarded because of the high impurity content.

Tables 3 and 4 below compare the purity of silica and ammonium fluoridesolution, respectively, obtained in accordance with this Example. Theyillustrate the superior product produced by the method of the invention.

                  TABLE 2                                                         ______________________________________                                        PURlTY OF AMMONIUM FLUOSILICATE                                               (QN 100 PERCENT SALT BASIS)                                                                 (NH.sub.4).sub.2 SiF.sub.6                                                                (NH.sub.4).sub.2 SiF.sub.6                          Element, ppm  Mother Liquor                                                                             Crystals                                            ______________________________________                                        Al            0.20        0.05                                                Ca            0.61        0.06                                                Cr            0.47        0.00                                                Fe            1.28        0.04                                                Mg            0.14        0.04                                                Mo            0.27        0.03                                                P             2.50        0.00                                                Pb            2.02        0.00                                                S             3545        0.81                                                Zn            0.20        0.00                                                Na            0.67        0.20                                                K             0.70        0.20                                                Li            0.70        0.20                                                ______________________________________                                         Ba, Cd, Co, Cu, Mn, Ni, and Sr are less than 0.05 ppm or not detected in      both cases.                                                              

                  TABLE 3                                                         ______________________________________                                        SlLICA PURITY                                                                                              Silica from                                                                   Ammonium                                                          Silica From Fluosilicate                                     Source           Mother Liquor                                                                             Crystals                                         ______________________________________                                        Element ppm                                                                   Al               0.36        0.04                                             Ca               0.64        0.14                                             Cr               0.08        0.01                                             Fe               2.22        0.26                                             Mg               0.00        0.05                                             Mo               0.00        0.00                                             P                0.12        0.30                                             Pb               0.15        0.00                                             S                6.54        0.48                                             Zn               0.11        0.03                                             Na               0.20        0.20                                             K                0.20        0.20                                             Li               0.20        0.20                                             Radioactive elements, ppb                                                     U                --          1                                                Th               --          --                                               ______________________________________                                         Ba, Cd, Co, Cu, Mn, Ni, and Sr are less than 0.05 ppm or not detected in      both cases.                                                              

                  TABLE 4                                                         ______________________________________                                        AMMONIUM FLUORIDE PURITY                                                      (40 PERCENT SOLUTION BASIS)                                                                             NH.sub.4 F from                                                               Ammonium                                                          NH.sub.4 F from                                                                           Fluosilicate                                        Source        Mother Liquor                                                                             Crystals                                            ______________________________________                                        Element, ppm                                                                  Al            0.44        0.05                                                Ca            0.00        0.02                                                Cr            0.24        0.01                                                Fe            1.04        0.02                                                Mg            0.06        0.02                                                Mo            0.20        0.01                                                P             1.88        0.18                                                Pb            1.28        0.00                                                S             1976        0.18                                                Zn            0.16        0.01                                                Na            0.40        0.10                                                K             0.40        0.10                                                Li            0.40        0.10                                                ______________________________________                                         Ba, Cd, Co, Cu, Mn, Ni, and Sr are less than 0.05 ppm or not detected in      both cases.                                                              

Although preferred embodiments of the invention have been discussedherein, those skilled in the art will appreciate that changes andmodifications may be made without departing from the spirit of thisinvention, as defined in and limited only by the scope of the appendedclaims. For example, those skilled in the art will recognize that impurefluosilicic acid from sources other than those discussed herein also maybe treated in accordance with the method of this invention to yield ahigh purity silica and ammonium fluoride.

I claim:
 1. A method for producing high purity silica and ammoniumfluoride comprising:(a) recovering silicon tetrafluoride-containing gasfrom the acidulation of a fluorine-containing phosphorus source; (b)separating liquid entrainment from the gas; (c) converting the gasrecovered from step (b) to an ammonium fluosilicate solution; and (d)ammoniating said ammonium fluosilicate solution to produce high puritysilica and ammonium fluoride.
 2. The method of claim 1 wherein saidfluorine-containing phosphorus source is a phosphorus-containing rockand said rock is acidulated with concentrated sulfuric acid.
 3. Themethod of claim 1 wherein the gas from step (b) is adsorbed in highpurity water to produce fluosilicic acid solution and silicaprecipitate, the fluosilicic acid solution being recovered separatelyfrom the silica precipitate.
 4. The method of claim 3 wherein theseparately-recovered fluosilicic acid solution is reacted with ammoniato yield the ammonium fluosilicate solution of step (c).
 5. The methodof claim 1 wherein ammonium fluosilicate crystals are recovered from theammonium fluosilicate solution from step (c), and said crystals areredissolved to yield an ammonium fluosilicate solution prior to theammoniation of step (d).
 6. The method of claim 4 wherein ammoniumfluosilicate crystals are recovered from the ammonium fluosilicatesolution from step (c), and said crystals are redissolved to yield anammonium fluosilicate solution prior to the ammoniation of step (d). 7.The method of claim 5 wherein the ammonium fluosilicate crystals arerecrystallized a plurality of times to purify the ammonium fluosilicatesolution.
 8. The method of claim 1 wherein the ammoniation of step (d)comprises contacting the aqueous solution of step (c) with ammonia at atemperature between about 25° and 85° C.
 9. The method of claim 8wherein the ammoniation of step (d) comprises contacting the aqueoussolution of step (c) with ammonia at a temperature between about 75° and85° C. until the ammonium fluosilicate concentration is less than about10 wt. percent, then reducing the temperature to between about 40°-60°C. for the remainder of the ammoniation.
 10. A method for producing highpurity silica and ammonium fluoride comprising:(a) recovering silicontetrafluoride-containing gas from the acidulation of afluorine-containing phosphorus source; (b) separating liquid entrainmentfrom the gas; (c) converting the gas recovered from step (b) to anammonium fluosilicate solution by absorbing the gas in a high purityammonium fluoride solution; and (d) ammoniating said ammoniumfluosilicate solution to produce high purity silica and ammoniumfluoride.
 11. A method for producing high purity silica and ammoniumfluoride comprising:(a) recovering silicon tetrafluoride-containing gasfrom the acidulation of a fluorine-containing phosphorus source; (b)separating liquid entrainment from the gas; (c) absorbing the gas fromstep (b) in high purity water to produce fluosilicic acid solution andsilica precipitate, the fluosilicic acid solution being recoveredseparately from the silica precipitate; (d) reacting the recoveredfluosilicic acid solution with ammonium fluoride to yield ammoniumfluosilicate solution; and (e) ammoniating said ammonium fluosilicatesolution to produce high purity silica and ammonium fluoride.
 12. Themethod of claim 10 wherein ammonium fluosilicate crystals are recoveredfrom the ammonium fluosilicate solution from step (c), and said crystalsare redissolved to yield and ammonium fluosilicate solution prior to theammoniation of step (d).
 13. The method of claim 11 wherein ammoniumfluosilicate crystals are recovered from the ammonium fluosilicatesolution from step (d), and said crystals are redissolved to yield anammonium fluosilicate solution prior to the ammoniation of step (e). 14.The method of claim 10 wherein said fluorine-containing phosphorussource is a phosphorus-containing rock and said rock is acidulated withconcentrated sulfuric acid.
 15. The method of claim 11 wherein saidfluorine-containing phosphorus source is a phosphorus-containing rockand said rock is acidulated with concentrated sulfuric acid.